In process automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. Besides sensors and actuators, referred to as field devices are generally also such units, which are connected directly to a fieldbus, and which serve for communication with a control unit such as a control system, i.e. such units as e.g. remote I/Os, gateway, linking devices and wireless adapters.
A large number of such field devices are produced and sold by the Endress+Hauser group of companies.
Frequently, field devices are connected by means of two wire technology to a control station. In the case of two wire technology, also called two conductor technology, electrical current for energy supply and for communication signals is sent over the same line: one wire for the outgoing direction, and one wire for the return path. In other words, energy supply and signal utilize the same line; there is no separate energy supply. This electrical current, or the corresponding power, must be managed by a field device and divided among the individual components of the field device. Thus, for instance, the sensor element, the communication and the control unit must manage together within the existing power budget.
The rejection of wired data transmission for connecting a field device has in the field of industry the potential to reduce costs of wiring, improve usability and therewith generate benefits for the user.
For energy supply, function modules such as, for instance, a sensor element M or a communication module, are connected after a direct voltage converter DC, which converts the input voltage from the two conductor bus 4 into a voltage suitable for the particular function module. See, in this connection, an example of an electronic circuit 2 from the state of the art in FIG. 1. For the measuring function in the case of certain measuring devices, such as, for instance, measuring the fill level using the radar principle, more power is consumed than the 4 . . . 20 mA supply can make available. Therefore, an energy storing capacitor C is necessary for storing energy for supplying the sensor element. One speaks also of buffering or of the buffer capacitor C. The storage capacitor C is charged between measuring phases directly from the two conductor bus 4. The measuring is only executed after the charge status of the storage capacitor C has reached a certain level. The charge status is measured, for instance, via the voltage V across the storage capacitor C. U.S. Pat. No. 7,262,693, for example, discloses the application of a capacitor, in order to store energy from the bus intermediately, in order then to provide it to a wireless module.
Likewise, a communication module could be present, which, in equal manner, such as above described, requires more power for operation, thus for sending/receiving, than is permanently and continuously available. This is, for example, the case for WLAN, for instance, according to a standard of the IEEE-802.11 family.
Other wireless communication modules, such as, for instance, Bluetooth with the protocol stack, Low Energy, as well as certain measuring functions could, in the present state of the art, manage completely with the power provided by the bus, even without any buffering. The two functions, measuring and communication, can, however, most often, not be operated simultaneously, since the power balance of the total device must be maintained. In many cases in the present state of the art, consequently, a buffering of the energy is applied for all function modules.